Microengineered self-releasing switch

ABSTRACT

A MEMS (microelectromechanical system) electrical switch device is provided for circuit protection applications. The device includes a mechanical latching mechanism by which the switch is held in the closed position, and a mechanism by which this latch is released when the load current passing through the device reaches or exceeds some desired magnitude. In addition, a mechanism is provided by which the switch may be reset to its closed position by applying an electrical control voltage to certain terminals of the device. A number of these devices, or arrays of these devices, can be fabricated by parallel processes on a single substrate, and photolithography can be employed to define the mechanical structures described above. Other embodiments include additional electrical isolation of the resetting mechanism, enhancement of the separation distance of the contact points of the switch in the open position, and prevention of arcing at the latch mechanism. A method of fabricating the device is provided. A method of using the aforementioned device is also provided.

FIELD OF THE INVENTION

The invention relates to switches and in particular to micro-engineeredswitches. More particularly the invention relates to types of switchesthat may self-release thereby enabling a circuit breaking functionalityto be incorporated within the switch.

BACKGROUND OF THE INVENTION

Electrical circuits are frequently connected to a source of power insuch a way that the connection is broken if the current drawn from thepower supply exceeds some previously determined level. The excessivecurrent demand will often be due to a failure in the system, such as ashort circuit being formed by the failure of a component. Isolating thecircuit in such a case may prevent or limit damage to the circuititself, the power supply, or associated systems, and reduce the risk offire or electrical shock. Isolation may be achieved by a fusible link or“fuse”, or by a reusable switch, generally known in this context as acircuit breaker. The fuse will generally be of lower cost, but must bephysically replaced in the event of an isolation incident occurring. Thecircuit breaker can be reset, usually through manual mechanicalactuation, and thus a single device can provide protection for a numberof incidents.

While solid state devices are the preferred option for many circuitisolation applications, for many others electromechanical switching ismore suitable. Electromechanical switches (usually magnetically actuatedrelays), offer low insertion loss, high current handling for a givensize due to reduced heat dissipation, a broader range of current vs.time response characteristics, ability to handle surges, and high opencircuit isolation. An example is U.S. Pat. No. 3,849,752. In this devicea thermally sensitive longitudinally expandable plunger is enclosed in aconductor casing connected in series with the circuit breaker switch. Inone embodiment the conductor casing has a suitable resistance forheating the plunger for tripping the circuit breaker at excessivecurrent levels.

Conventional circuit breakers have also been described which are used tobreak a current path in the case of a general thermal overloadcondition, rather than at a designated current load. An example isprovided by U.S. Pat. No. 6,154,116, which describes a thermal circuitbreaker and switch. In that invention, a bimetallic element is employedto effect the tripping of the switch when an overload condition isreached. Resetting is done for both the aforementioned devices via amanually operated actuator.

Improved manufacturing techniques have allowed traditional relay designsto achieve much lower cost at higher reliability. Never-the-less, boththe size and cost of circuit breakers currently manufactured areexcessive for some applications, and the sophistication of theiroperation is limited. Thus there is a need for improved designs ofcircuit breakers, intended to handle modest current levels, that providecomplex functionality, low cost and low overall size. One possibilityfor achieving such designs is to take advantage of the manufacturingtechniques of the semiconductor industry, particularly parallelmanufacturing of large numbers of components on single substrates, andthe parallel definition of complex structures by photolithography. Morespecifically, the opportunity exists to use the manufacturing technologyof micro-electro-mechanical systems, or “MEMS”. MEMS technology usesmanufacturing techniques developed by, or similar to those used in, thesemiconductor micro-electronics industry. This approach is naturallysuited to sub-miniature relays, offering high functional complexity atlow manufacturing cost, and improved integration of electromechanicalfunctions with solid-state electronics.

Electrical MEMS relays and switches are known in the art. For example,patent No. WO9950863 describes a micromachined relay including aspringing beam on which a magnetic actuation plate is formed. By thepresence or absence of a magnetic field, the springing beam is bent soas to open or close a pair of electrical contacts, so creating anelectrical short circuit or open circuit. With this or other similardevices, it would be possible to implement circuit protection usingexternal current sensing and circuitry to obtain a trip signal by whichthe micromachined relay could be opened. However, protection should notbe dependent on the proper working of external circuits, and thus thetrip action should be intrinsic to the relay mechanism. Also, a separatecurrent sensing mechanism would be required which did not itself havesome undesired effect on the power supply, the load or the system as awhole.

U.S. Pat. No. 5,463,233 describes a micromachined thermal switch. Inthis invention a bimetallic plate is provided which bends according toits temperature, such as to make electrical contact between a pair ofterminals for a certain temperature range. In that invention, additionalelectrostatic actuation forces are provided so as to give the switch asnap action and thus reduce arcing when the gap between the fixed andmoving parts of the electrical path is small. Here the temperature ofthe bimetallic plate is not controlled via the switched current.

In U.S. Pat. No. 6,211,598, a MEMS thermal actuator is provided, whichgives in-plane mechanical motion by the use of a composite member havingdifferent degrees of thermal expansion. In that invention the heating ofthe composite beam may be effected by a mechanism intrinsic to thedevice. In that invention no means is provided by which the actuator maybe employed to break the electrical path of the current by which theactuating heat is provided.

In Xi-Qing Sun, K. R. Farmer, W. N. Carr, Proc. IEEE MEMS Workshop,1998, a bi-stable MEMS relay is reported in which a cantilever is heldin the closed position by a mechanical catch mechanism. Closing andopening of the relay switch is effected by applying voltages in thecorrect -sequence to each of two thermal actuation structures comprisingthe cantilever, such that the cantilever deforms so as to achieve boththe closing or opening and the latching or unlatching. The intendedapplication is given as switching of high frequency signals, and noprovision is made for opening of the switch in response to the switchedload current.

A laterally moving thermal MEMS actuator is described in Comtois &Bright, Sensors & Actuators A58(1), 1997, using two element cantileversin which one element heats preferentially when a current is passedthrough the device. The applications described are for motors andoptical structures.

Micromachined MEMS devices have been described which use electrostaticforces to operate electrical switches and relays. Typically in thesedevices, cantilever beams separated from the underlying substrate haveelectrical contacts at their free ends, such that these contacts move asthe cantilever deflects, so that electrical connections may be made orbroken to additional contacts fixed on the substrate. For example, U.S.Pat. Nos. 5,367,136, 5,258,591, and 5,268,696 to Buck et al., U.S. Pat.No. 5,544,001 to Ichiya, et al., and U.S. Pat. No. 5,278,368 to Kasano,et al. are representative of this class of MEMS switch and relaydevices. More specifically, U.S. Pat. No. 6,229,683 describes a highvoltage micromachined switch. In that invention, a composite beam isdeflected by electrostatic forces, and in this way electrical connectionis made or broken between electrical contacts fixed on a substrate. Inaddition, features are incorporated including the use of multiplecontacts, and electrically isolated contacts, so as to reduce thepossibility of arc formation when high voltages are applied to thedevice in its open state. The control of the device, however, remainsfunctionally separate from the electrical path which is switched, andthus an intrinsic circuit protection function is not provided.

A further example of a cantilevered switch device is described inInternational application WO 02/17339. This specification describes MEMSswitches using pairs of cantilevered, thermally driven actuators. Thecontrol of this device is functionally separate from the electrical pathwhich is switched. Furthermore, the construction of the switch is suchthat a sequence of steps requiring an actuation of both cantilevers isrequired to effect an adoption of one of the two states of the switch.

These and other problems associated with the prior art means that thereis a need for an electromechanical switch that can be re-settable in anefficient manner.

It is therefore an object of the present invention to provide are-settable electromechanical circuit breaker switch, suitable forfabrication by MEMS technology.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a micro-electromechanicalswitch comprising:

-   -   a substrate;    -   first and second conductive cantilevers on the substrate;    -   the second conductive cantilever being flexible with respect to        the first from a rest position at which the two are mechanically        and electrically isolated to a latched position at which they        are mechanically latched to form an electrical connection, the        latched position being effected on application of a control        current through the second cantilever;    -   the first conductive cantilever being flexible with respect to        the second from a corresponding latched position to a release        position at which the two are no longer mechanically latched;        and    -   a first current path passing through the said electrical        connection such that the passage of a first threshold electrical        current through the first current path causes the first        cantilever to flex from its latched position to its release        position, thus breaking the said electrical connection and        allowing the second conductive cantilever to return to its rest        position.

It will be appreciated that the electrical connection formed through thelatching of the first and second cantilevers may or may not be aconnection between the individual cantilevers but could be a connectionthrough one of the cantilevers and a third component.

Preferably, the switch further comprises a second current pathassociated with the second cantilever such that the passage of a secondthreshold electrical current through the second current path causes thesecond cantilever to flex from its rest position to its latchedposition. This enables the switch to be reset by the application of anelectrical current.

The switch is preferably fabricated by fabricating a base for attachmentof the cantilevers on a first level, fabricating moving parts of thecantilevers on a second level and fabricating electrical contacts of thecantilevers on the second level or a third level, wherein each level isformed by the deposition and patterning of a sacrificial layer that isused as a mould for the fabrication of the conductive parts.

In another embodiment, the invention provides an microelectromechanicalswitch comprising:

-   -   a substrate;    -   a first cantilever attached to, but electrically isolated from,        the substrate, having two longitudinal segments, the first of        which is electrically connected at the fixed end to a first        primary electrical terminal and the other of which is        electrically connected at the fixed end to an additional        terminal, and the two members being attached to each other at a        point along their lengths;    -   a second cantilever attached to, but electrically isolated from,        the substrate, having two isolated longitudinal segments, each        of which is electrically connected at the fixed end to a        secondary terminal, and the two members being attached to each        other at a point along their lengths, and the cantilever having        an electrical contact point at its distal end;    -   a fixed electrical contact point attached to, but electrically        isolated from, the substrate, and electrically connected to a        second primary electrical terminal; and    -   wherein the two cantilevers are not in mechanical contact when        in their relaxed state.

Desirably each of the two cantilevers have attached mechanical partswhich cause them to be mechanically linked together when one is deformedsuch as to bring it in contact with the other, and this linkage is suchthat the cantilevers remain in contact when the actuation force whicheffected this contact is removed.

When the two cantilevers are in contact, the two contact points (that onthe second cantilever and that on the fixed part) are typically held incontact, and consequently a low resistance electrical path is obtainedbetween the first and second primary terminals.

Desirably, the passing of an electrical current between the primaryterminals, when the two cantilevers are latched together, causes heatingof the first longitudinal segment of the first cantilever in such a wayas to induce its deformation, and wherein this deformation issufficient, on the passing of certain such currents for sufficient time,to cause the latching mechanism to be released, and the two cantileversto separate, and so causing interruption of the low resistance pathbetween the primary terminals.

Typically, the actuation of the second cantilever to bring it intocontact with, and cause it to be latched to, the first cantilever, canbe effected by the passing of a current between the two secondaryterminals, by the effect of differential thermal expansion associatedwith heating of the cantilever by the passed current.

An additional electrical path may be provided between the primaryterminals through the second longitudinal element of the firstcantilever, and wherein the relative amount of current flowing throughthe two electrical paths may be determined by the connection of aresistor between the two terminals of the first cantilever, and whereinthe total current required to release the latch mechanism may so bealtered.

The second cantilever desirably includes an additional mechanicalstructure such that the relative motion of the electrical contact pointbetween the latched and relaxed states of the cantilever issubstantially greater than the relative motion of the moving end of thecantilever at the point at which this mechanism is attached.

An additional actuator may be provided which is electrically isolatedfrom the two cantilevers, and wherein this actuator may be used to bringthe two cantilevers into contact and to cause them to be latchedtogether, by the mechanical contact of this actuator against one of thecantilevers. The additional actuator is desirably in the form of a thirdcantilever, having two isolated longitudinal segments, each of which iselectrically connected to a terminal, and wherein the passing of acurrent between the terminals of this third cantilever causes themovement of the actuator as required.

According to a further embodiment of the invention a method offabrication of an electromechanical switches is also provided in whichbase parts for the cantilevers are fabricated on one level, thecantilevers on a further level, and the contact points on a furtherlevel, each level being formed by the deposition and patterning of asacrificial polymer layer, for example photoresist, and this layer beingused as a mold for the electroplating of the parts in metal, the polymerlayers being subsequently removed.

According to a further embodiment a device consisting of a packagecontaining a single die cut from a substrate on which electromechanicalswitches have been fabricated is provided, wherein more than one suchswitch is included on the die, and is thus accessible from the terminalsof the package.

The invention may also provide an application wherein aelectromechanical switch or array of switches connected between one ormore circuits requiring protection and one or more voltage sources isprovided, such that the connection between any of the circuits requiringprotection and any voltage source is disconnected if the current betweenthem exceeds a certain level for a certain duration of time. Such anapplication may also include a control circuit which monitors the stateof the protected terminals, and possibly also monitors aspects of thestate of the protected circuits, and according to the its programmingapplies voltages to the switch or switches at the appropriate terminalsso as to reset the corresponding switch and thus reestablish electricalconnection of the relevant protected circuit and voltage source.

The substrate of the switching device may includes an electronic circuitwhich is electrically connected to the switching device and wherein thiselectronic circuit provides or contributes to some control function ofthe switch or some related function.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example withreference to the accompanying drawings, in which:

FIG. 1A is a plan view of a preferred embodiment of the invention in theopen state, showing the two cantilevered structures, the latchmechanism, the electrical terminals and the electrical contact points ofthe switch.

FIG. 1B shows the same mechanism in the closed state, with onecantilevered structure in a non-relaxed state held in contact with theother by the latch mechanism.

FIG. 2 shows an external electrical connection used to prevent arcing atthe latch mechanism, by keeping both sides of the latch at the samepotential.

FIG. 3 shows the use of an external resistor to increase the tripcurrent.

FIG. 4A and 4B show the invention in open and closed state,respectively, with the addition of a feature to magnify the displacementof the moving contact point, to increase the degree of isolation in theopen state.

FIG. 5 shows a variation of the invention in which the resettingmechanism is electrically separate from the path of the primary current.

FIG. 6 illustrates a preferred fabrication technique for the invention.

FIG. 7 illustrates an array of devices suitable for packaging as asingle device.

FIG. 8 illustrates a potential use of the device in the protection of anelectronic system; and

FIG. 9 shows a variation of the invention in which the electricalconnection is made by bringing one contact against both parts of a splitcontact.

DETAILED DESCRIPTION

Referring in detail to the drawings where similar parts are identifiedby like reference numbers, there is seen in FIGS. 1A and 1B a diagram ofa circuit breaking switch in its open and closed position respectively.The switch is mounted in such a way as to present a number of electricalterminals 160 for connection to a printed circuit board or otherwise forconnection to external electrical circuits. These terminals could be forexample the pins of a standard semiconductor package of a type usedconventionally for mounting of integrated circuits on printed circuitboards. Terminals 160 d and 160 e provide the power supply and loadconnection respectively, such that the purpose of the switch is toprevent excess current levels from flowing from the supply to the load.Each external terminal 160 is electrically connected, for example bywire bonding, to a fixed anchor 150, each anchor being a partmechanically fixed to, but electrically isolated from, the substrate.The substrate may be a silicon wafer, or some other planar substratesuitable for processing with semiconductor process equipment such asphotolithography tools.

A first cantilever 200 a has two parallel members, 110 a and 120 a,which are mechanically and electrically connected to anchors 150D and150C respectively at their proximal ends. The parallel members 110 a and120 a are connected to each other mechanically and electrically at somedistal point, which may be the distal end of one or both of them,although one or both may extend beyond this connection point. Thecantilever includes or comprises an electrically conductive layer suchthat a low resistance electrical path is provided, running sequentiallythrough members 110 a and 120 a, from anchor 150D to 150C.

A second cantilever 200 b includes two parallel members 110 b and 120 b,which are connected to, and provide an electrical path between, anchors150A and 150B in the manner of the equivalent members of the firstcantilever. Anchors 150A and 150B are connected to external terminals160 a and 160 b respectively, in the manner of the connection of anchors150 a and 150 b to their corresponding terminals. The second cantileverincludes a flexible member 170 connected at its distal end whichprovides a low resistance electrical path from the connection point ofmembers 110 b and 120 b to an electrical contact, 180 a. A secondelectrical contact 180 b is attached to a further anchor 150E which isitself connected to a terminal 160 e in the manner previously described.

Both cantilevers 200 a and 200 b have attached latch parts, 130 a and130 b respectively. In the open position of the switch, as illustratedin FIG. 1A, there is no electrical or mechanical connection between thecantilevers 200 a and 200 b, and no electrical or mechanical connectionbetween the contacts 180 a and 180 b. The two cantilevers 200 a and 200b, including the members 110, the latch parts 130 and the part 170, arefabricated such that they are not in mechanical contact with thesubstrate underneath them, and are only mechanically connected to theanchors 150, and thus indirectly to the substrate.

A current may be passed through cantilever 200 b by the application of asuitable voltage between terminals 160 a and 160 b. As a result of theelectrical resistance of members 110 b and 120 b, such a current causesheating of these members. This heating causes the members to increase inlength. The members are fabricated in such a way that the increase inlength experienced by member 110 b is greater than that experienced by120 b. This may be achieved by member 110 b being narrower than 120 b,so that its resistance is greater. The difference in length increasewill cause the cantilever 200 b to bend, such that the two contacts 180a and 180 b come into contact with each other, and the catch mechanismscontact each other. With application of the correct current forsufficient time, the cantilever 200 b will bend past the point where thecontacts 180 a and 180 b meet, such that member 170 bends, and the catchparts 130 a and 130 b engage with each other.

Upon engagement of the catch parts, the cantilevers 200 a and 200 bremain mechanically locked together even after the current betweenterminals 160 a and 160 b ceases, such that the contacts also remainheld together by a force resulting from the bending of member 170. A lowresistance electrical path is now provided between the primary terminals160 d and 160 e. This path passes through member 110 a, through thecatch parts 130 a and 130 b, through member 170 and through the contacts180 a and 180 b. Current flowing through this path (the “load” current)causes heating of member 110 a, which consequently expands in length andcauses cantilever 200 a to bend. This bending causes the catch parts 130a and 130 b to begin to separate, such that when the desired tripcurrent is reached for sufficient time, the catch releases, andcantilever 200 b returns to its relaxed position. As a consequence ofthis movement, the electrical path between the primary terminals isbroken.

With reference to FIG. 2, it may be desirable to prevent the formationof an electrical arc between the catch parts during the setting of theswitch. This avoids the fabrication of catch parts that can withstandsuch arcing. This can be achieved as illustrated, by providing anelectrical connection between terminals 160 c and 160 b. This connectionmay be within the device as packaged, or provided by external circuitry.With such a connection effected, the two catch parts are kept at thesame voltage, and an additional current path between the primaryterminals is provided, passing consecutively through members 110 a, 120a, 120 b and thence member 170 and the contacts as before. Withreference now to FIG. 3, it may also be desirable for the user to varythe load current at which the switch trips (the ‘trip current’s). Aconvenient possibility for the user is to connect a resistor betweenterminals of the device, and this resistor should be, for convenience,of a higher resistance than that of the low resistance path through theswitch in its closed state. Such a possibility is obtained through thecorrect design of cantilever 200 a. A resistor 210 is connected betweenterminals 160 d and 160 e. This allows part of the load current totravel sequentially through this resistor and through member 120 a, inparallel to the path through member 110 a. This reduces the differencein lengthening between members 110 a and 120 a as the load currentincreases, and thus increases the total load current required to tripthe switch. By varying the relative dimensions of members 110 a and 120a, the range of trip current values and the corresponding resistancevalues for resistor 210 can be varied.

Referring now to FIG. 4, it may be desirable to increase the separationof the contacts 180 a and 180 b in the open state of the switch, toincrease the voltage necessary to cause an arc to form between them, andit may also be desirable to increase the speed at which these contactsseparate at the moment of tripping. This can be provided by the additionof a part 220 to cantilever 200 b. This part makes mechanical contactwith an additional fixed anchor 150F when the switch is closed, in sucha way that member 230 of this part 220 rotates by the bending of anadditional member 240. By this construction the total displacement ofthe moving contact 180 a, and therefore its velocity during tripping andits maximum separation from contact 180 b, are increased.

Referring now to FIG. 5, it may be advantageous to increase the securityof the circuit breaking function, to reduce the possibility that theload connected to terminal 160 e may obtain power from the externalcircuitry connected to terminal 160 b, for example as a result offailure in that circuitry. Such a current path would reduce the currentthrough member 110 a, and therefore increase the trip current. Thisincreased security is achieved by the addition of a third cantilever250, which includes the catch part 130 b and the member 170 and theattached contact 180 a, but does not require the two parallel memberstructure. It is connected to a single anchor and terminal, 150 f and160 f respectively. The resetting cantilever now no longer requires theparts 130 a, 170 and 180 a. Resetting of the switch is performed bypassing a current between terminals 160 a and 160 b as describedpreviously. FIG. 5(b) shows this switch in the state of being reset.When this setting current ceases, cantilevers 200 a and 200 c remain inmechanical and electrical contact with each other, while cantilever 200b returns to its rest position. In this state there is no electricalpath between the circuits connected to cantilever 200 b and the primaryterminals 160 d and 160 e.

The cantilevers, anchors and contacts may be fabricated on a substrateusing sacrificial layer processing, as is well known in the art. Anexample is given in FIG. 6, in which a possible approach to fabricationof one of the cantilevers 200 is shown by way of illustration. Asemiconducting substrate such as silicon is coated with an insulatinglayer, such as silicon dioxide, of a thickness and quality able towithstand the maximum voltage from which the external load is to beprotected without becoming conducting or damaged. A thin metal layersuch as copper is applied, for example by physical vapour deposition, tothe surface of the insulating layer to act as a seed layer forsubsequent electroplating. A first layer of polymer is applied, andpatterned by photolithography to provide base layers for the anchors150. The polymer may be photoresist which is directly patterned byphotolithography. It may also be another polymer, such as polyimide,onto which a layer of photoresist is applied, this photoresistsubsequently being patterned by photolithography and then used as amasking layer for patterning, for example by reactive ion etching, ofthe first polymer layer, after which the photoresist layer is removed,for example by dissolution in a solvent. The first polymer layer, havingbeen patterned, is subsequently used as a mold for electroplating of thebase using a suitable metal, for example copper.

The process of seed layer deposition, polymer patterning andelectroplating is repeated in a further layer, at which level the mainparts of the cantilevers 200, catch parts and other parts arefabricated. This layer will also be of a suitable metal, for examplecopper. The electrical contacts 180 a and 180 b may be fabricated on athird layer by a similar set of process steps, so as to be attached tothe associated parts 170 and 150E respectively with some overlap in thedesired area of contact. This layer may be fabricated by two sequencesof polymer mold patterning and electroplating, so that the contacts maybe of two different compositions, for example two gold alloys, in orderto reduce the likelihood of the contacts fusing together duringoperation. All polymer layers are removed, leaving only the metal partsattached to the insulating layer on the substrate. The processesdescribed above would be carried out on a whole wafer on which a numberof devices would be fabricated in parallel. The wafer would then bediced into a number of individual components, which would then bepackaged using packaging techniques and formats as known in the art forpackaging of integrated circuits and other electrical and electroniccomponents, particularly for mounting on circuit boards. Mounting of theindividual dies could also be onto multi-chip modules, by bump bondingor other suitable techniques known in the art.

An alternative embodiment is also possible in which the mechanical partsare defined in a layer of silicon, for example a single crystal siliconlayer bonded to a silicon wafer with an intervening oxide layer, astructure known in the art as bonded silicon on insulator (BSOI). Theoxide layer would then provide the functions of an anchor, electricalisolation of the switch parts from the substrate, and a sacrificiallayer for release of the moving parts from the substrate. A process ofthis type for forming MEMS devices from BSOI is described in Syms R. R.A., et al, Sensors and Actuators, vol. 88/3, pp. 273-283. The siliconlayer could be heavily doped to provide high conductance, or additionalmetal layers could be deposited to provide a low resistance currentpath. Additional metal layers could also be deposited to form thecontacts 180.

The device of the present invention is also suitable for fabrication inarray form. In this case each die would include more than one switch,and would be packaged as a single package. FIG. 7 illustrates a die withtwo switches; the extension of this layout to larger numbers of switcheswill be obvious to one skilled in the art by inspection of this figure.This package would in general have separate terminals for each suchswitch, as indicated in the figure, where 160al indicates terminal 160 afor the first switch, and so on. Some sharing of terminals, in order toreduce the package size, is also possible. For example, if connectionbetween terminals 160 b terminals 160 c as in FIG. 2 are not to beemployed, the anchor points 150 b from all the switches could beconnected to a single terminal 160 b.

FIG. 8 illustrates an application of the present invention. A packageddevice 400 containing 2 switches,.with terminals for each switchlabelled as in FIG. 7, is mounted on a printed circuit board 430. Powerto the circuit board is provided at two voltages with respect to earth(eg. +12V and −12V), at terminals labelled V1 and V2, with the earthterminal labelled 0V, although the extension of this application exampleto a single voltage or to larger numbers of supply voltages will beobvious by reference to this figure to one skilled in the art. Anapplication circuit represented by block 410 requires power from thesupply voltages, but also needs protection from excessive currents. Thisis provided by connecting 410 to the supply terminals via the circuitprotection device 400. An additional control circuit 420, which may be asingle digital integrated circuit, is also mounted on the circuit board430. This control circuit is directly powered from one or more of thesupply terminals, and monitors terminals 160 e 1 and 160 e 2 to sensewhether a trip event has occurred. It also may monitor one or moreterminals 450 on the application circuit 410 to sense desired aspects ofthe state of this circuit.

According to the state of the sensed lines, and according to the logicin its design or programming, control circuit 420 will apply voltages toreset lines 160 a 1 and 160 a 2 as appropriate to reset the switches.The application circuit may also have additional connections 440 toother circuits, components or terminals on or off the circuit board. Inaddition, the switch parts could be fabricated on a substrate havingelectronic circuitry on it, and this circuitry could include all or partof the control circuit function, thus providing a monolithic device withboth the electromechanical and the control circuit functions on a singlechip.

With reference to FIG. 9, it may be desirable to avoid the need for theprotected current to flow through member 170, as this member may be ofexcessive electrical resistance. This can be achieved as illustrated, byreplacing the single fixed contact 180 b with a split contact having twoparts 180 b and 180 c. These parts 180 b and 180 c are attached,respectively, to anchors 150E and 150F, which are in turn connected,respectively, to terminals 160 e and 160 f in the manner previouslydescribed.

In this embodiment, in the state in which the first and secondcantilevers are latched together, the moving contact 180 a makeselectrical contact with both parts 180 b and 180 c of the split contact.As illustrated, an electrical connection can also be provided betweenterminals 160 d and 160 f. This connection may be within the device aspackaged, or provided by external circuitry. With such a connectioneffected, terminals 160 c and 160 e act as the primary terminals of thedevice, and the load current passes sequentially through the firstcantilever and through the two contact parts 180 b and 180 c via contact180 a.

Many modifications and other embodiments of the invention will come tomind to one skilled in the art to which this invention pertains havingthe benefit of the teachings presented in the foregoing descriptions andthe associated drawings. Therefore, it is to be understood that theinvention is not to be limited to the specific embodiments disclosed andthat modifications and other embodiments are intended to be includedwithin the scope of the appended claims. Although specific terms areemployed herein, they are used in a generic and descriptive sense onlyand not for purposes of limiting the scope of the present invention inany way.

It will be appreciated that the present invention provides a switch thatopens, or “trips”, when the load current exceeds a desired value formore than a desired time, as a result of a mechanism intrinsic to theswitch device of the present invention. It will be further understoodthat the switch device of the present invention may be closed, or“reset”, by the application of an electrical current. These and otherfeatures of the present invention have been described with reference topreferred micromechanical devices that include a substrate onto whichare attached conductive parts which, when in contact with each other,provide a low resistance path between two primary terminals. These partsinclude two or more cantilevered structures which are mechanically fixedto the substrate at one end, but are free to move at the other end whendeformed, in a motion primarily parallel to the surface of thesubstrate. When they are in their relaxed state, these structures arenot in electrical contact with each other, and a high resistance obtainsbetween the primary terminals. Deformation is caused by differentialthermal expansion, where as a result of a difference in temperaturechange or thermal expansion coefficient between different parts of astructure, heating causes the structure to bend. The low resistancecontact is made by one of the structures being deformed in this way,such that the two structures previously not in electrical contact comeinto contact, and the two structures are held in contact by a latchingmechanism. Passing of a current (the “Load current”) between the primaryterminals is possible when (and only when) the two structures are incontact, and the device is constructed such that the load current mustpass through one of the cantilevered structures in such a way thatdeformation by differential thermal expansion is caused as a result ofheating due to ohmic resistance. The structures are configured such thatwhen this deformation exceeds an amount corresponding to the desiredtrip current, the latch is released and the cantilevered structuresseparate, breaking the low resistance path between the primaryterminals.

Resetting of the device of the present invention is desirably effectedby the application of a current between two resetting terminals, one ofwhich may be common to one of the primary terminals. This current causesdeformation of one of the cantilevered structures by the methodpreviously described, in such a way as to bring the two cantileveredstructures into contact, and to latch them together. This resettingfunction may be provided by a current in one of the load currentcarrying structures, or it may be applied to an additional structurewhich is electrically isolated from the load current carryingstructures, but which moves one of these by mechanical contact in orderto achieve the resetting function. This additional structure providesadditional protection, by ensuring that the behaviour (including thefailure) of any circuitry connected to the resetting terminals does notprovide an alternative current path for the load, or otherwise alter theelectrical behaviour of the load current carrying portion of the presentinvention. As such, it will be appreciated that the present invention isnot intended to be limited in any way except as may be deemed necessaryin the light of the appended claims. References U.S. Pat. No. 3849752-Bayer; Eric W., U.S. Pat. No. 5258591- Buck; Daniel C., U.S. Pat. No.5268696- Buck; Daniel C., Grice; Steven, U.S. Pat. No. 5278368- Kasano;Fumihiro, Nishimura; Hiromi, et al U.S. Pat. No. 5367136-- Buck; DanielC., U.S. Pat. No. 5463233- Norling; Brian L., U.S. Pat. No. 5544001-Ichiya; Mitsuo et al U.S. Pat. No. 6154116- Sorenson; Richard W., U.S.Pat. No. 6211598- Dhuler; Vijayakumar R., et al U.S. Pat. No. 6229683-Goodwin-Johansson; Scott Halden WO9950863- Tai, Yu-Chong, Wright, John,A. WO 02/17339 JDS Uniphase Corp.Xi-Qing Sun, K. R. Farner, W. N. Carr, Proc. IEEE MEMS Workshop, 1998,154-159 Syms R. R. A., Gomley C., Blackstone S. “Improving yield,accuracy and complexity in surface tension self-assembled MOEMS” Sensorsand Actuators, 88/3,273-283 (2001) Comtois J H, Bright V M,“Applications for surface-micromachined polysilicon thermal actuatorsand arrays”, Sensors & Actuators A58(1), 1997, pp. 19-25

1. A micro-electromechanical switch comprising: a substrate; first andsecond conductive cantilevers on the substrate; the second conductivecantilever being flexible with respect to the first from a rest positionat which the two are mechanically and electrically isolated to a latchedposition at which they are mechanically latched to form an electricalconnection, the latched position being effected on application of acontrol current through the second cantilever; the first conductivecantilever being flexible with respect to the second from acorresponding latched position to a release position at which the twoare no longer mechanically latched; and a first current path passingthrough the said electrical connection such that the passage of a firstthreshold electrical current through the first current path causes thefirst cantilever to flex from its latched position to its releaseposition, thus breaking the said electrical connection and allowing thesecond conductive cantilever to return to its rest position.
 2. Amicro-electromechanical switch according to claim 1 to which: the firstcantilever comprises two elongate conductive members mechanicallyattached to one another at a point along their lengths; and the firstcurrent path passes through at least one of the elongate conductivemembers of the first cantilever, such that the passage of the firstthreshold electrical current through the first current path causesdifferential thermal expansion of the elongate conductive members, thuscausing the first cantilever to flex.
 3. A micro-electromechanicalswitch according to claim 2 in which: the elongate conductive membersare of substantially the same electrical resistivity and thermalexpansivity; the first current path passes through both of the elongateconductive members of the first cantilever; and passage of the firstthreshold electrical current through the first current path gives riseto different current densities in the two elongate conductive members,thus causing differential thermal expansion.
 4. Amicro-electromechanical switch according to claim 3 in which the firstcurrent path passes through the elongate conductive members of the firstcantilever in parallel and further comprising a resistor coupled intoone of the parallel current paths thus created to determine the value ofthe first threshold electrical current.
 5. A micro-electromechanicalswitch according to claim 1 in which the first current path passesthrough the second cantilever to an electrical contact thereon andthence to a fixed electrical contact on the substrate.
 6. Amicro-electromechanical switch according to claim 5 in which theelectrical contact on the second cantilever and the fixed electricalcontact on the substrate are mechanically and electrically isolated whenthe second cantilever is in its rest position and are in mechanical andelectrical contact when the second cantilever is in its latchedposition.
 7. A micro-electromechanical switch according to claim 5 inwhich the second cantilever includes a lever arrangement thatexaggerates the movement of the electrical contact on the secondcantilever as the second cantilever is flexed from its rest position toits latch position.
 8. A micro-electromechanical switch according toclaim 1 in which the first cantilever is a unitary component.
 9. Amicro-electromechanical switch according to claim 1 further comprising;a second current path associated with the second cantilever such thatthe passage of a second threshold electrical current through the secondcurrent path causes the second cantilever to flex from its rest positionto its latched position.
 10. A micro-electromechanical switch accordingto claim 9 in which: the second cantilever comprises two elongateconductive members mechanically attached to one another at a point alongtheir lengths; and the second current path passes through at least oneof the elongate conductive members of the second cantilever, such thatthe passage of the second threshold electrical current through thesecond current path causes differential thermal expansion of theelongate conductive members, thus causing the second cantilever to flex.11. A micro-electromechanical switch according to claim 10 in which: theelongate conductive members of the second cantilever are ofsubstantially the same electrical resistivity and thermal expansivity;the second current path passes through both of the elongate conductivemembers of the second cantilever; and passage of the second thresholdelectrical current through the second current path gives rise todifferent current densities in the elongate conductive members, thuscausing differential thermal expansion.
 12. A micro-electromechanicalswitch according to claim 9 in which the second cantilever is a unitarycomponent.
 13. A micro-electromechanical switch according to claim 9 inwhich the second cantilever is a two-part component comprising: a firstcomponent through which the first current path passes: and a secondcomponent through which the second current path passes.
 14. Amicro-electromechanical switch according to claim 1 in which the twocantilevers include respective resilient deformable latching projectionsthat resiliently latch one another as the second cantilever flexes fromits rest position to its latched position and disengage from each otheras the first conductive cantilever flexes from its latched position toits release position.
 15. A micro-electromechanical switch according toclaim 1 comprising a plurality of such first cantilevers and acorresponding plurality of such second cantilevers, forming a pluralityof independent switches on a common substrate.
 16. Amicro-electromechanical switch according to claim 15 in the form of apackage containing a single die and wherein the electrical current pathsfor each switch are accessible from the terminals of the package.
 17. Amicro-electromechanical switch according to claim 1 further comprising asplit contact, the split contact having two legs, a first leg beingcoupled to the first cantilever thereby providing a current path fromthe first cantilever to the split contact and wherein on adoption of thelatched position by the second cantilever, a current path is providedbetween the first and second legs of the split contact, therebyelectrically coupling the second leg to the first cantilever.
 18. Amicro-electromechanical switch according to claim 17 wherein the currentpath between the first and second legs is provided via a contactprovided on an end portion of the second cantilever.
 19. (canceled) 20.A device comprising: a power source; one or more circuits requiringprotection; a micro-electromechanical switch according to claim 9connected between the power source and the one or more circuitsrequiring protection; and, a control circuit adapted to pass the secondthreshold electrical current through a selected second current path toestablish an electrical connection between a corresponding circuitrequiring protection and the power source in accordance withpredetermined conditions.
 21. A device comprising amicro-electromechanical switch according to claim 1 and an electroniccircuit provided on the substrate and connected to the switch. 22.(canceled)
 23. A method of fabricating a micro-electromechanical switchaccording to claim 1 comprising: fabricating a base for attachment ofthe cantilevers on a first or level; fabricating moving parts of thecantilevers on a second level; and, fabricating electrical contacts ofthe cantilevers on the second level or a third level, wherein each levelis formed by the deposition and patterning of a sacrificial layer thatis used as a mould for the fabrication of the conduct parts.
 24. Amethod according to claim 23 in which the sacrificial layer is apolymeric photoresist.
 25. A method according to claim 23 or claim 24 inwhich the conductive parts are metallic parts formed by electroplating.26. (canceled)